Method for forming compressive alpha-tantalum on substrates and devices including the same

ABSTRACT

A layer of compressive alpha-tantalum is formed on a substrate by depositing a buffer layer on the substrate and depositing a layer of compressive alpha-tantalum on the buffer layer with lattice matching between the layer of compressive alpha-tantalum and the buffer layer. In some embodiments, the method may include depositing buffers layers comprising titanium, niobium, substantially pure aluminum or aluminum-copper alloy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is related to a copending and simultaneouslyfiled utility patent application Ser. No. 10/426,561 titled “FLUIDEJECTION DEVICE WITH COMPRESSIVE ALPHA-TANTALUM LAYER,” filed Apr. 29,2003.

BACKGROUND OF THE INVENTION

Tantalum (Ta) thin films are widely used in manufacturing ofsemiconductor and micro-electromechanical systems (MEMS). For example,in semiconductor integrated circuit manufacturing, tantalum may be usedas a diffusion barrier between copper and silicon. Tantalum may also beused as a gate electrode in metal oxide semiconductor field effecttransistor (MOSFET) devices. Tantalum may also be used to absorb X-raysin X-ray masks. In thermal inkjet MEMS such as a printhead, tantalum isused as a protective overcoat on the resistor and other substrate layersto protect the underlying layers from damage caused by cavitation fromthe collapsing ink bubbles. The tantalum layer also protects theunderlying layers of a printhead from chemical reactions with the ink.

The metastable tetragonal phase of tantalum, known as the beta-phase or“beta-tantalum” is typically used in the manufacture of thermal inkjetdevices. This beta-tantalum layer is brittle and becomes unstable astemperatures increase. Above 300° C., beta-tantalum converts to thebody-centered-cubic (bcc) alpha-phase or “alpha-tantalum.”Alpha-tantalum is the bulk equilibrium or stable-phase of tantalum. Itis desired to form stable, compressive alpha-tantalum films and suchfilms on fluid ejection devices. Such compressive alpha-tantalum filmsmay increase the useful life of such devices by resistance to peeling,blistering or delamination from the substrate.

SUMMARY OF THE INVENTION

A layer of compressive alpha-tantalum is formed on a substrate bydepositing a buffer layer on the substrate and depositing a layer ofcompressive alpha-tantalum on the buffer layer with lattice matchingbetween the layer of compressive alpha-tantalum and the buffer layer. Insome embodiments, the method may include depositing buffers layerscomprising titanium, niobium, substantially pure aluminum oraluminum-copper alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings illustrate exemplary embodiments for carrying outthe invention. Like reference numerals refer to like parts in differentviews or embodiments of the drawings.

FIG. 1 is a flow chart of a method of forming a layer of compressivealpha-tantalum on a substrate according to an embodiment of the presentinvention.

FIG. 2 is a cross-sectional graphical representation of a compressivealpha-tantalum thin film according to an embodiment of the presentinvention.

FIG. 3 is a cross-sectional graphical representation of a fluid ejectiondevice including compressive alpha-tantalum according to an embodimentof the present invention.

FIG. 4 is a graph of X-ray diffraction data corresponding to acompressive alpha-tantalum film with titanium buffer layer grownaccording to an embodiment of the present invention.

FIG. 5 is a graph of X-ray diffraction data corresponding to acompressive alpha-tantalum film with niobium buffer layer grownaccording to an embodiment of the present invention.

FIG. 6 is a graph of X-ray diffraction data corresponding to acompressive alpha-tantalum film with a substantially pure aluminumbuffer layer grown according to an embodiment of the present invention.

FIG. 7 is a graph of X-ray diffraction data corresponding to acompressive alpha-tantalum film with an aluminum-copper alloy bufferlayer grown according to an embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the invention include a method of forming a layer ofcompressive alpha-tantalum on a substrate. Compressive alpha-tantalumthin films, fluid ejection devices, thermal inkjet printheads andthermal inkjet printers are also disclosed. Reference will now be madeto exemplary embodiments illustrated in the drawings, and specificlanguage will be used herein to describe the same. It will neverthelessbe understood that no limitation of the scope of the invention isthereby intended. Alterations and further modifications of the inventivefeatures illustrated herein, and additional applications of theprinciples of the inventions as illustrated herein, which would occur toone skilled in the relevant art and having possession of thisdisclosure, are to be considered within the scope of the invention.

A thermal inkjet (TIJ) printhead typically includes a silicon substratehaving conductive and resistive layers thereon to provide electricalfeatures that are used to heat and eject ink from the printhead. Theresistive layers are used to heat ink until it vaporizes, creating abubble. The expansion of the ink vapor forms a bubble that ejects theink out from the printhead as an ink drop onto a target, typicallypaper, as a single dot or pixel. The term “firing” as used hereincontemplates the whole process of heating of the ink and ejecting theink as an ink drop and the collapse of the ink vapor bubble.

Problems associated with conventional TIJ printheads include failuresresulting from high thermo-mechanical stresses caused during and afterthe firing of the ink drop, mechanical shock generated by the collapseof the ink bubble (cavitation) and the corrosive nature of the ink. Forthese reasons, protective layers are typically placed over the resistorand other layers forming the printhead to prolong the life of theprinthead.

Resistive elements (sometimes referred to herein as heating elements) ona printhead substrate are typically covered with a passivation layer,e.g., silicon nitride (SiN), and/or silicon carbide (SiC) and acavitation barrier layer, e.g., tantalum. Silicon nitride is ceramicmaterial and an electrical insulator that protects the resistor fromelectrically shorting. Silicon carbide is a hard semiconductor materialand structurally amorphous. Silicon carbide is used to prevent ink frompermeating through and reaching the underlying layers of a printhead andto provide mechanical robustness. Tantalum has good mechanical strengthto withstand the thermo-mechanical stresses that result from theejection of the ink. Additionally, tantalum has chemical inertness atelevated temperatures that minimizes corrosion caused by ink.

The tantalum layer is often composed of the metastable tetragonal phaseof tantalum, known as the beta-phase or “beta-tantalum.” Thisbeta-tantalum layer is brittle and becomes unstable as temperaturesincrease.

FIG. 1 is a flow chart of a method 100 of forming a layer of compressivealpha-tantalum on a substrate according to embodiments of the presentinvention. The substrate may be formed of a semiconductor material. Thesubstrate may include other layers of materials including siliconnitride (SiN) and/or a layer of silicon carbide (SiC). The siliconcarbide layer may be on the surface of the substrate. Method 100 mayinclude depositing 102 a buffer layer on the substrate and depositing104 a layer of compressive alpha-tantalum on the buffer layer withlattice matching between the layer of compressive alpha-tantalum and thebuffer layer. The layer of compressive alpha-tantalum may have thicknessranging from about 10 Angstroms (A) to about 4 micrometers (μm).

The term “lattice matching” refers to when lattice points of crystalplanes of materials forming a common interface approximately match eachother geometrically across their interface. For two distinct crystalplanes to match geometrically across their interface the symmetries ofthese planes are substantially identical and their lattice mismatcheswithin less than about 5% of each other. Lattice matching is alsodefined in Strained Layer Superlattices, Semiconductors and Semimetals,Vol. 33, edited by R. K. Willardson and A. C. Beer (Academic, New York,1990) and also in J. A. Venables, G. Spiller, and M. Hanbucken, Rep.Prog. Phys. 47, 399 (1984) and references cited therein.

Depositing 102 the buffer layer and depositing 104 the compressivealpha-tantalum may be performed using any suitable physical vapordeposition technique. For example, and not by way of limitation,sputtering, laser ablation, e-beam and thermal evaporation techniques,individually or in combination, may be used in depositing 102 and 104.Depositing 102 and 104 may be performed at any temperature includingsubstrate temperatures less than 300° C. Furthermore, depositing 102 thebuffer layer may further include application of a substrate voltagebias. The voltage bias may range from about 0 volts to about −500 volts,using a conventional DC magnetron sputtering process.

Depositing 102 a buffer layer may include depositing a layer oftitanium. The layer of titanium may have a thickness from about 3monolayers to about 2000 Å according to embodiments of the presentinvention. Presently preferred thicknesses for titanium buffer layersmay range from at least about 400 Å according to other embodiments ofthe present invention. For atomically smooth substrate surfaces, thelayer of titanium is contemplated to be as thin as a single monolayer inaccordance with embodiments of the present invention. In one embodiment,the layer of titanium may orient on the substrate with titanium crystal[100] direction perpendicular to the substrate. According to anotherembodiment, lattice matching may occur between the layer of titanium andthe layer of compressive alpha-tantalum.

Depositing 102 a buffer layer may include depositing a layer of niobium.The layer of niobium may have a thickness from about 3 monolayers toabout 2000 Å consistent with embodiments of the present invention. Foratomically smooth substrate surfaces, the layer of niobium iscontemplated to be as thin as a single monolayer in accordance withembodiments of the present invention. Presently preferred thicknessesfor niobium buffer layers may range from at least about 200 Å accordingto other embodiments of the present invention.

In another embodiment, depositing 102 a buffer layer may includedepositing a layer of substantially pure aluminum or aluminum-copperalloy. The layer of aluminum-copper alloy may include up to about 10% byweight of copper. The layer of substantially pure aluminum oraluminum-copper alloy may have a thickness from about 3 monolayers toabout 2000 Å consistent with embodiments of the present invention. Foratomically smooth substrate surfaces, the layer of substantially purealuminum or aluminum-copper alloy is contemplated to be as thin as asingle monolayer in accordance with embodiments of the presentinvention.

FIG. 2 is a cross-sectional graphical representation of a compressivealpha-tantalum thin film stack 200 according to embodiments of thepresent invention. The compressive alpha-tantalum thin film stack 200may include a ceramic material 204 in contact with a substrate 202, abuffer layer 206 in contact with the ceramic material 204 and acompressive alpha-tantalum layer 208 lattice matched to the buffer layer206. The ceramic material 204 may include silicon carbide (SiC). Thebuffer layer may include at least one of titanium, niobium,substantially pure aluminum and aluminum-copper alloy.

FIG. 3 is a cross-sectional graphical representation of a fluid ejectiondevice 300 including compressive alpha-tantalum according to embodimentsof the present invention. The fluid ejection device 300 may comprise athermal inkjet printhead or thermal inkjet printer consistent withembodiments of the present invention. The fluid ejection device 300 mayinclude a substrate stack 301. The substrate stack 301 may include aresistive element 306, bulk substrate 302, an optional capping layer304, an insulating ceramic material 308 and a ceramic material 310. Thefluid ejection device 300 may further include a buffer layer 312 formedon the second ceramic material 310 and a compressive alpha-tantalumlayer 314 lattice matched to the buffer layer 312.

The capping layer 304 may include, for example and not by way oflimitation, a thermal oxide, silicon dioxide (SiO₂), ortetraethylorthosilicate (TEOS) layer. The buffer layer 312 is in contactwith second ceramic material 310. Likewise, buffer layer 312 is incontact with the compressive alpha-tantalum 314. The insulating ceramicmaterial 308 may include silicon nitride (SiN). Second ceramic material310 may include silicon carbide (SiC). The buffer layer 312 may beformed on second ceramic material 310 by at least one of the followingphysical vapor deposition techniques: sputtering, laser ablation, e-beamand thermal evaporation. The layer of compressive alpha-tantalum 314 mayhave a thickness ranging from about 10 Å to about 4 μm. In accordancewith embodiments of the present invention, the buffer layer 312 may beformed of any material that forces tantalum to grow in a compressivestate as alpha-tantalum, through, for example, lattice matching. In someembodiments, the buffer layer is at least one of titanium, niobium,substantially pure aluminum and aluminum-copper alloy as furtherexplained below with reference to the examples.

EXAMPLE 1 Titanium Buffer Layer

In this embodiment, the buffer layer 312 may be formed of a layer oftitanium. The layer of titanium may have a thickness ranging from about3 monolayers to about 2000 Å according to embodiments of the presentinvention. As mentioned above, presently preferred thicknesses fortitanium buffer layers may range from at least about 400 Å according toother embodiments of the present invention. The crystal structure oftitanium is hexagonal closed packed (hcp). In one embodiment of thepresent invention, the layer of titanium may orient on a substrate stack301 with the titanium crystal [100] direction perpendicular to thesubstrate stack 301. In another embodiment, the layer of titanium mayinclude textured titanium grains.

The tantalum overlayer orients in Ta[110] direction perpendicular to thesubstrate with compressive residual stress. Lattice matching across theTi/Ta interface forces the tantalum overlayer to grow in the bodycentered cubic (bcc) alpha-tantalum phase.

Table 1, below, shows parameters taken from five study wafers 1–5 withtitanium buffer layers and compressive alpha-tantalum overlayers inaccordance with the method of embodiments of the present invention. Eachwafer included a bulk silicon substrate with passivation layers ofsilicon nitride and silicon carbide. For each wafer, the buffer layer oftitanium was first sputter deposited onto the silicon carbide surfacefollowed by sputtering of the compressive alpha-tantalum layer. Columns2–3 of Table 1 show tantalum/titanium (Ta/Ti) layer thicknesses measuredin Å and alpha-tantalum film stress measured in Mega-Pascals (MPa).Columns 4–5 show deposition parameters for each tantalum layer, i.e.,argon flow rate measured in SCCM (flow of Standard gas at a pressure ofone atmosphere at a rate of one Cubic Centimeter per Minute) and argonpressure measured in millitorrs (mTorr), respectively. Column 6 showsplasma power applied during sputter deposition measured in kilo-Watts(kW). Plasma power was reduced from 3 kW to 1.5 kW for thinner layers oftitanium to increase the precision in thickness control. The titaniumlayers were grown at an argon pressure of 2.5 mTorr and an argon flowrate of 100 SCCM. Of course, one skilled in the art will recognize thatthe plasma power ranges, argon pressure and flow rate stated above forthese particular embodiments are merely exemplary and that other rangesand settings for these parameters are also within the scope of thepresent invention.

TABLE 1 Alpha- Ta/Ti Tantalum Argon Argon Layer Film Flow PressurePlasma Thicknesses Stress Rate (in units Power Wafer (in units (in units(in units of (in units No. of Å) of MPa) of SCCM) mTorr) of kW) 13000/100 −651.4 100 5 10 (Ta)/ 1.5 (Ti) 2 3000/200 −747.1 100 5 10 (Ta)/1.5 (Ti) 3 3000/400 −744.8 100 5 10 (Ta)/ 3 (Ti) 4 3000/600 −730.4 100 510 (Ta)/ 3 (Ti) 5 3000/800 −706.8 100 5 10 (Ta)/ 3 (Ti)

Another aspect of embodiments of the present invention includingtitanium buffer layers is the internal or residual stresses in theresultant alpha-tantalum thin film. The underlying substrate layers,such as silicon nitride (SiN) and silicon carbide (SiC) are undercompressive stresses. For this additional reason, the alpha-tantalumoverlayer is grown in compression to substantially avoid blistering anddelamination.

In this embodiment, the alpha-tantalum films of Table 1 were grown undercompressive stress. No voltage biasing was applied to the substrateduring deposition. However, in some embodiments, applying a substratevoltage bias would make the alpha-tantalum thin films even morecompressive if desired. The tantalum and titanium layers were depositedusing DC magnetron sputtering according to embodiments of the presentinvention. However, other physical vapor deposition techniques may beused consistent with other embodiments of the present invention, forexample and not by way of limitation, laser ablation, e-beam and thermalevaporation.

The strength of adhesion of the Ta/Ti bilayer to the silicon carbidepassivation layer was tested using a Scotch™ tape method. The Scotch™tape was used to attempt to peel off the Ta/Ti bilayer from the siliconcarbide passivation layer. The Ta/Ti bilayer failed to peel off. In oneembodiment, the strong adhesion between Ta/Ti bilayer and the siliconcarbide passivation layer may result from the formation of titaniumcarbide (TiC) covalent bonds across the SiC/Ti interface that providesstrong bonding between the SiC/Ti interface layers. Furthermore, thebonds between the compressive alpha-tantalum topcoat and its titaniumbuffer layer are metallic.

FIG. 4 is a graph of X-ray diffraction data corresponding to acompressive alpha-tantalum film with titanium buffer layer grown onstudy wafer number 2 according to method 100 of the embodiments of thepresent invention. In FIG. 4, the x-axis is diffraction angle measuredin angular degrees and the y-axis is intensity measured in arbitraryunits. The compressive alpha-tantalum was deposited on a 200 Å thicklayer of titanium. The peak corresponds to [110] orientedalpha-tantalum. The inset graph shows vertical lines drawn to indicatethe expected peak positions for beta-Ta(002) and alpha-Ta(200)reflections. Both of these expected reflections are absent, indicating awell-oriented alpha-Ta(110) layer grown on study wafer number 2. Sincethe peaks for alpha-Ta(110) and its Ti(100) buffer layer overlap, thereflection peak for titanium is masked and, thus, does not appear inFIG. 4. Additionally, the number of diffraction lines in the x-ray scansshown in FIG. 4 reveal [110] oriented single-phase alpha-tantalumoverlayer, slight asymmetry apparent in the diffraction peaks may beattributed to an unreacted [001]-textured titanium buffer layer.

Table 2, below, shows X-ray diffraction data for the study wafers 1–5 ofTable 1. Columns 2–6 show tantalum/titanium layer thicknesses in unitsof Å, tantalum phase, alpha-tantalum lattice spacing in units of Å,tantalum grain size in units of Å and alpha-tantalum rocking curve asmeasured in angular degrees at Full Width of the peak at Half Maximumpeak height (FWHM). The width of the rocking curve provides a measure ofthe orientational distribution of alpha-tantalum columnar grains inangular degrees. The tantalum grain size and rocking curve data indicatethat the titanium buffer layers with thicknesses 200 Å, 400 Å and 600 Åprovide the desirable, larger tantalum grain size, i.e., approximately130 angstroms, with narrower grain orientation distribution.

TABLE 2 Alpha- Alpha- Ta/Ti Tantalum Tantalum Layer Lattice TantalumRocking Thicknesses Spacing Grain Size Curve (in Wafer (in unitsTantalum in units (in units units of° No. of Å) Phase of Å) of Å) FWHM)1 3000/100 alpha 3.340 ± ~100 5.4 0.001 2 3000/200 alpha 3.343 ± ~1303.8 0.001 3 3000/400 alpha 3.343 ± ~130 3.9 0.001 4 3000/600 alpha 3.341± ~130 3.8 0.001 5 3000/800 alpha 3.340 ± ~120 4.1 0.001

EXAMPLE 2 Niobium Buffer Layer

In this embodiment, the buffer layer 312 may be formed of a layer ofniobium. The layer of niobium may have a thickness ranging from about 3monolayers to about 2000 Å. As mentioned above, presently preferredthicknesses for niobium buffer layers may range from at least about 200Å according to other embodiments of the present invention. Niobium andtantalum are members of the same column of the periodic table ofelements and have similar physical properties. The crystal structure ofniobium is bcc, which is the same as alpha-tantalum. The tantalum (110)overlayer almost perfectly lattice matches on the Nb(110) plane sincethe lattice spacings of the alpha-tantalum and niobium are almostidentical, i.e., 3.3026 Å and 3.3007 Å, respectively. Unlike tantalumhowever, niobium does not grow in the beta-phase structure. Niobiumalways grows in the alpha-phase structure irrespective of the presenceof impurity gases on the substrate or the substrate material type.Because of this property, if a thin layer of niobium is first depositedon a substrate stack 301, the tantalum overlayer is forced to grow inthe alpha-tantalum phase because of lattice matching across thetantalum/niobium interface.

Table 3, below, shows parameters taken from six study wafers 6–11 withniobium buffer layers and compressive alpha-tantalum overlayers inaccordance with the embodiments of the present invention. Each waferincluded a bulk silicon substrate with passivation layers of siliconnitride and silicon carbide. For each wafer, the buffer layer of niobiumwas first sputter deposited onto the silicon carbide surface followed bysputtering of the compressive alpha-tantalum layer. The niobium layerthickness for the study wafers varied from 25 to 800 Å. Columns 2–3 ofTable 3 show Ta/Nb layer thicknesses measured in Å and alpha-tantalumfilm stress measured in MPa. Columns 4–5 show deposition parameters forthe tantalum layer, i.e., argon flow rate measured in SCCM, argonpressure measured in mTorr, respectively. Column 6 shows plasma powerduring sputter deposition measured in kW for tantalum and niobiumlayers, respectively. According to another embodiment of the presentinvention, thinner layers of niobium may be obtained by reducing theplasma power to about 0.5 kW, thus, allowing greater precision inthickness control. According an embodiment of the present invention, theniobium buffer layers were grown at an argon pressure of 2.5 mTorr andan argon flow rate of 100 SCCM. Of course, one skilled in the art willrecognize that the above-stated plasma power ranges, argon pressure andflow rate for these particular embodiments are merely exemplary and thatother ranges and settings for these parameters are also within the scopeof the present invention.

TABLE 3 Alpha- Ta/Nb Tantalum Argon Argon Layer Film Flow PressurePlasma Thicknesses Stress Rate (in units Power Wafer (in units (in units(in units of (in units No. of Å) of MPa) of SCCM) mTorr) of kW) 63000/25   −1529.9 100 5 10 (Ta)/ 1 (Nb) 7 3000/50  −1477.5 100 5 10(Ta)/ 1 (Nb) 8 3000/100 −1477.9 100 5 10 (Ta)/ 1 (Nb) 9 3000/200 −1404.5100 5 10 (Ta)/ 1 (Nb) 10 3000/400 −1267.8 100 5 10 (Ta)/ 1 (Nb) 113000/800 −1024.8 100 5 10 (Ta)/ 1 (Nb)

Another aspect of embodiments of the present invention including niobiumbuffer layers is the internal or residual stresses in the resultantalpha-tantalum thin film. The stress data shown in Table 3 indicatesthat the alpha-tantalum films were grown under compressive stress.Additionally, the alpha-tantalum film stresses show a dependence on thethickness of the niobium buffer layer. No voltage biasing was applied tothe substrate during deposition. Applying a substrate voltage biascauses the alpha-tantalum thin films to be even more compressiveaccording to other embodiments of the present invention. The tantalumand niobium layers were deposited using DC magnetron sputteringaccording to embodiments of the present invention. However, otherphysical vapor deposition techniques may be used consistent with otherembodiments of the present invention, for example and not by way oflimitation, laser ablation, e-beam and thermal evaporation.

The strength of adhesion of the Ta/Nb bilayer to the silicon carbidepassivation layer was tested using a Scotch™ tape method. The Scotch™tape was used to attempt to peel off the Ta/Nb bilayer from the siliconcarbide passivation layer. The Ta/Nb bilayer failed to peel off. In oneembodiment, the adhesion strength can be attributed to metallic bondingsbetween tantalum and its niobium buffer layer. In another embodiment,alloying of niobium and silicon, forming NbSi covalent bonds across theSiC/Nb interface may ensure robust bonding of these layers together. Seefor example, M. Zhang et al., Thin Solid Films, Vol. 289, no. 1–2, pp.180–83 and S. N. Song, et al., Journal of Applied Physics, Vol. 66, no.11, pp. 5560–66.

FIG. 5 is a graph of X-ray diffraction data corresponding to acompressive alpha-tantalum film with a niobium buffer layer grown onstudy wafer number 6 according to method 100 of the embodiments of thepresent invention. In FIG. 5, the x-axis is diffraction angle measuredin angular degrees and the y-axis is intensity measured in arbitraryunits. The compressive alpha-tantalum layer was deposited on a 25 Åthick layer of niobium. The peak corresponds to [110] orientedalpha-tantalum. The inset graphs show vertical lines drawn to indicatethe expected peak position for beta-Ta(002) reflection. Additionally,the main graph shows an arrow indicating the expected peak position foran alpha-Ta(200) reflection. Both of these expected reflections areabsent, indicating a well-oriented alpha-Ta(110) layer grown on studywafer number 6. In FIG. 5, the expected niobium reflection is maskedbecause the peaks for alpha-Ta(110) and its Nb(110) buffer layeroverlap.

Table 4, below, shows X-ray diffraction data for study wafers 6–11 ofTable 1. Columns 2–6 show tantalum/niobium layer thicknesses in units ofÅ, tantalum phase, alpha-tantalum lattice spacing in units of Å,tantalum grain size in units of Å and alpha-tantalum rocking curve asmeasured in angular degrees at FWHM. The tantalum grain size and rockingcurve data shown in Table 4 indicate that the 800 Å thickness niobiumbuffer layer provides a larger tantalum grain size with narrower grainorientation distribution with smaller internal stress relative to studywafers 6–10, see also Table 3.

TABLE 4 Alpha- Alpha- Ta/Nb Tantalum Tantalum Layer Lattice TantalumRocking Thicknesses Spacing Grain Size Curve (in Wafer (in unitsTantalum in units (in units units of° No. of Å) Phase of Å) of Å) FWHM)6 3000/25  alpha 3.337 ± ~160 4.3 ± 0.2 0.001 7 3000/50  alpha 3.336 ±~160 4.4 ± 0.2 0.001 8 3000/100 alpha 3.336 ± ~170 4.3 ± 0.2 0.001 93000/200 alpha 3.336 ± ~175 4.3 ± 0.2 0.001 10 3000/400 alpha 3.335 ±~180 4.3 ± 0.2 0.001 11 3000/800 alpha 3.334 ± ~190 4.0 ± 0.2 0.001

EXAMPLE 3 Substantially Pure Aluminum Buffer Layer

In this embodiment, the buffer layer 312 may be formed of a layer ofsubstantially pure aluminum. The buffer layer may also be alloyed withcopper, see Example 4, below. The crystal structure of aluminum is facecentered cubic (fcc) and lattice matches on the Al(111) plane with theTa(110) plane. Because of this property, if a thin layer ofsubstantially pure aluminum is first deposited on a substrate stack 301,the tantalum overlayer is forced to grow in the alpha-phase because oflattice matching across the tantalum/substantially pure aluminum (Ta/Al)interface.

Table 5, below, shows parameters taken from five study wafers 12–16 withsubstantially pure aluminum buffer layers and compressive alpha-tantalumoverlayers in accordance with of embodiments of the present invention.Each of the study wafers 12–16 included a bulk silicon substrate withpassivation layers of silicon nitride and silicon carbide. For eachwafer, the buffer layer of substantially pure aluminum was first sputterdeposited onto the silicon carbide surface followed by sputtering of thecompressive alpha-tantalum layer. The substantially pure aluminum layerthickness for the study wafers 12–16 varied from 100 to 800 Å accordingto embodiments of the present invention. Columns 2–3 of Table 5 showTa/Al layer thicknesses measured in Å and alpha-tantalum film stressmeasured in MPa. Columns 4–5 show deposition parameters for thetantalum-layer, i.e., argon flow rate measured in SCCM, argon pressuremeasured in mTorr, respectively. Column 6 shows plasma power duringsputter deposition measured in kW for tantalum and substantially purealuminum layers, respectively. The substantially pure aluminum bufferlayers were grown at an argon pressure of 2.5 mTorr and an argon flowrate of 50 SCCM according to embodiments of the present invention. Ofcourse, one skilled in the art will recognize that the above-statedplasma power ranges, argon pressure and flow rate for these particularembodiments are merely exemplary and that other ranges and settings forthese parameters are also within the scope of the present invention.

TABLE 5 Alpha- Ta/Al Tantalum Argon Argon Layer Film Flow PressurePlasma Thicknesses Stress Rate (in units Power Wafer (in units (in units(in units of (in units No. of Å) of MPa) of SCCM) mTorr) of kW) 123000/100 −1022.4 50 5 5 (Ta)/5 (Al) 13 3000/200 −1020.2 50 5 5 (Ta)/5(Al) 14 3000/400 −1005.5 50 5 5 (Ta)/5 (Al) 15 3000/600 −906.5 50 5 5(Ta)/5 (Al) 16 3000/800 −908.0 50 5 5 (Ta)/5 (Al)

Another aspect of embodiments of the present invention includingsubstantially pure aluminum buffer layers is the internal or residualstresses in the resultant alpha-tantalum thin film. The stress data(column 3) shown in Table 5 indicates that the alpha-tantalum films weregrown under compressive stress. The compressive stress in thealpha-tantalum grown on the substantially pure aluminum buffer layerscan be attributed to the substantially pure aluminum buffer layer.Because of lattice matching across the tantalum/substantially purealuminum interface, the alpha-tantalum overlayer is forced to grow incompressive stress. Additionally, the alpha-tantalum film stresses showa dependence on the thickness of the substantially pure aluminum bufferlayer. No voltage biasing was applied to the substrate duringdeposition. Applying a substrate voltage bias causes the alpha-tantalumthin films to be even more compressive according to other embodiments ofthe present invention. The tantalum and substantially pure aluminumlayers were deposited using DC magnetron sputtering according toembodiments of the present invention. However, other physical vapordeposition techniques may be used consistent with other embodiments ofthe present invention.

The strength of adhesion of the Ta/Al bilayer to the silicon carbidepassivation layer was tested using a Scotch™ tape method. The Scotch™tape was used to attempt to peel off the Ta/Al bilayer from the siliconcarbide passivation layer. The Ta/Al bilayer failed to peel off. In oneembodiment, the adhesion strength can be attributed to metallic bondingsbetween tantalum and its aluminum buffer layer and bond formationsacross the SiC/Al interface, ensuring robustness of the adhesion betweenthese layers.

FIG. 6 is a graph of X-ray diffraction data corresponding to acompressive alpha-tantalum film with a substantially pure aluminumbuffer layer grown on study wafer number 14 according to the method 100of the embodiments of the present invention. In FIG. 6, the x-axis isdiffraction angle measured in angular degrees and the y-axis isintensity measured in arbitrary units. The compressive alpha-tantalumlayer was deposited on a 400 Å thick layer of substantially purealuminum. The peak corresponds to [110] oriented alpha-tantalum. Theinset graphs show vertical lines drawn to indicate the expected peakposition for beta-Ta(002) reflections. Additionally, the main graphshows an arrow indicating the expected peak position for analpha-Ta(200) reflection. Both of these expected reflections are absentor small, indicating a well-oriented alpha-Ta(110) layer grown on studywafer number 18. The expected Al(111) reflection is masked because thepeaks for alpha-Ta(110) and its Al(111) buffer layer overlap.

Table 6, below, shows X-ray diffraction data for the study wafers 12–16of Table 1. Columns 2–6 show tantalum/substantially pure aluminum layerthicknesses in units of Å, tantalum phase, alpha-tantalum latticespacing in units of Å, tantalum grain size in units of A andalpha-tantalum rocking curve as measured in angular degrees at FWHM. Thetantalum grain size and rocking curve data shown in Table 6 indicatesthat the 800 Å thick substantially pure aluminum buffer layer providesnarrower grain orientation distribution with smaller internal stressrelative to the other study wafers, see Table 5.

TABLE 6 Alpha- Alpha- Ta/Al Tantalum Tantalum Layer Lattice TantalumRocking Thicknesses Spacing Grain Size Curve (in Wafer (in unitsTantalum in units (in units units of° No. of Å) Phase of Å) of Å) FWHM)12 3000/100 alpha 3.329 ± ~115 20 ± 1  0.001 13 3000/200 alpha 3.330 ±~110 16 ± 1  0.001 14 3000/400 alpha 3.330 ± ~105 13 ± 1  0.001 153000/600 alpha 3.331 ± ~105  12 ± 0.5 0.001 16 3000/800 alpha 3.330 ±~100 9.5 ± 0.5 0.001

EXAMPLE 4 Aluminum-Copper Alloy Buffer Layer

In this embodiment, the buffer layer 312 may be formed of a layer of analuminum-copper alloy. The layer of aluminum-copper alloy may include upto about 10% by weight of copper and the balance substantially purealuminum. Al—Cu alloys are frequently used in the integrated circuit(IC) industry rather than substantially pure aluminum because Al—Cu isless susceptible to electromigration induced failures. Additionally,substantially pure aluminum targets used for sputtering are moreexpensive and less readily available than Al—Cu targets. As noted above,the crystal structure of aluminum is face centered cubic (fcc) andlattice matches on the Al(111) plane with the Ta(110) plane. Because ofthis property, if a thin layer of aluminum-copper alloy is firstdeposited on a substrate stack 301, the tantalum overlayer is forced togrow in the alpha-phase because of lattice matching across thetantalum/aluminum-copper alloy interface. Furthermore, the crystalstructure of copper is fcc and copper impurity atoms in aluminum latticewould occupy and substitute for Al atoms at fcc sites.

Table 7, below, shows parameters taken from six study wafers 17–22 withaluminum-copper alloy buffer layers and compressive alpha-tantalumoverlayers in accordance with method 100 of the embodiments of thepresent invention. The Al—Cu alloy targets used for study wafers 17–22each had up to about 5% by weight of copper with the balancesubstantially pure aluminum. Each wafer included a bulk siliconsubstrate with passivation layers of silicon nitride and siliconcarbide. For each wafer, the buffer layer of aluminum-copper alloy wasfirst sputter deposited onto the silicon carbide surface followed bysputtering of the compressive alpha-tantalum layer. The aluminum-copperalloy layer thicknesses for the study wafers 17–22 varied from 100 to800 Å according to embodiments of the present invention. Columns 2–3 ofTable 7 show Ta/Al—Cu layer thicknesses measured in Å and alpha-tantalumfilm stress measured in MPa. Columns 4–5 show deposition parameters forthe tantalum layer, i.e., argon flow rate measured in SCCM, argonpressure measured in mTorr, respectively. Column 6 shows plasma powerduring sputter deposition measured in kW for tantalum andaluminum-copper alloy layers, respectively. The aluminum-copper alloybuffer layers were grown at an argon pressure of 5 mTorr and an argonflow rate of 100 SCCM according to embodiments of the present invention.Of course, one skilled in the art will recognize that the above-statedplasma power ranges, argon pressure and flow rate for these particularembodiments are merely exemplary and that other ranges and settings forthese parameters are also within the scope of the present invention.

TABLE 7 Alpha- Ta/Al—Cu Tantalum Argon Argon Layer Film Flow PressurePlasma Thicknesses Stress Rate (in units Power Wafer (in units (in units(in units of (in units No. of Å) of MPa) of SCCM) mTorr) of kW) 173000/100 −450.1 100 5 10 (Ta)/ 1 (Al—Cu) 18 3000/200 −614.2 100 5 10(Ta)/ 1 (Al—Cu) 19 3000/300 −666.5 100 5 10 (Ta)/ 1 (Al—Cu) 20 3000/400−615.6 100 5 10 (Ta)/ 1 (Al—Cu) 21 3000/600 −556.8 100 5 10 (Ta)/ 1(Al—Cu) 22 3000/800 −507.6 100 5 10 (Ta)/ 1 (Al—Cu)

Another aspect of embodiments of the present invention includingaluminum-copper alloy buffer layers is the internal or residual stressesin the resultant alpha-tantalum thin film. The stress data (column 3)shown in Table 7 indicates that the alpha-tantalum films were grownunder compressive stress. The compressive stress in the alpha-tantalumgrown on the aluminum-copper alloy buffer layers can be attributed tothe aluminum-copper alloy buffer layer. Because of lattice matchingacross the tantalum/aluminum-copper alloy interface, the alpha-tantalumoverlayer is forced to grow in compressive stress. No voltage biasingwas applied to the substrate during deposition. Applying a substratevoltage bias causes the alpha-tantalum thin films to be even morecompressive according to other embodiments of the present invention. Thetantalum and aluminum-copper alloy layers were deposited using DCmagnetron sputtering according to embodiments of the present invention.However, other physical vapor deposition techniques may be usedconsistent with other embodiments of the present invention.

The strength of adhesion of the Ta/Al—Cu bilayer to the silicon carbidepassivation layer was tested using a Scotch™ tape method. The Scotch™tape was used to attempt to peel off the Ta/Al—Cu bilayer from thesilicon carbide passivation layer. The Ta/Al—Cu bilayer failed to peeloff. In one embodiment, the adhesion strength can be attributed tometallic bonds between tantalum and its aluminum buffer layer and acrossthe SiC/Al—Cu interface, ensuring robustness of the adhesion betweenthese layers.

FIG. 7 is a graph of X-ray diffraction data corresponding to acompressive alpha-tantalum film with aluminum-copper alloy buffer layergrown on study wafer number 18 according to method 100 of theembodiments of the present invention. In FIG. 7, the x-axis isdiffraction angle measured in angular degrees and the y-axis isintensity measured in arbitrary units. The compressive alpha-tantalumlayer was deposited on a 200 Å thick layer of aluminum-copper alloy. Thepeak in the main graph corresponds to [110] oriented alpha-tantalum. Theinset graph shows a vertical line drawn to indicate the expected peakposition for Al(200) reflections. Additionally, the main graph shows anarrow indicating the expected peak position for an alpha-Ta(200)reflection. Both of these expected reflections are absent or small,indicating a well-oriented alpha-Ta(110) layer grown on study wafernumber 18. The expected Al(111) reflection is masked because the peaksfor alpha-Ta(110) and its Al(111) buffer layer overlap.

Table 8, below, shows X-ray diffraction data for the study wafers 17–22of Table 7. Columns 2–6 show tantalum/aluminum-copper alloy layerthicknesses in units of Å, tantalum phase, alpha-tantalum latticespacing in units of Å, tantalum grain size in units of A andalpha-tantalum rocking curve as measured in degrees FWHM. As shown inTable 8, the tantalum thin films on wafers 17–22 exhibit diffusely orbroadly distributed grains.

TABLE 8 Alpha- Alpha- Ta/Al—Cu Tantalum Tantalum Layer Lattice TantalumRocking Thicknesses Spacing Grain Size Curve (in Wafer (in unitsTantalum in units (in units units of° No. of Å) Phase of Å) of Å) FWHM)17 3000/100 alpha & 3.321 ± ~105 ∞ beta 0.001 18 3000/200 alpha 3.324 ±~110 ∞ 0.001 19 3000/300 alpha 3.324 ± ~115 ∞ 0.001 20 3000/400 alpha3.323 ± ~115 ∞ 0.001 21 3000/600 alpha 3.323 ± ~110 ∞ 0.001 22 3000/800alpha 3.323 ± ~110 ∞ 0.001

It is to be understood that the above-referenced arrangements andexamples are illustrative of the applications for the principles ofembodiments of the present invention. Numerous modifications andalternative arrangements may be devised without departing from thespirit and scope of embodiments of the present invention. Whileembodiments of the present invention have been shown in the drawings anddescribed above in connection with the exemplary embodiments of theinvention, it will be apparent to those of ordinary skill in the artthat numerous modifications may be implemented without departing fromthe principles and concepts of the invention as set forth in the claims.

1. A method of forming a layer of compressive alpha-tantalum on asubstrate, comprising: applying a voltage bias to the substrate whiledepositing a buffer layer on the substrate; and depositing the layer ofcompressive alpha-tantalum on the buffer layer with lattice matchingbetween the layer of compressive alpha-tantalum and the buffer layer. 2.The method according to claim 1, wherein depositing comprises at leastone of the following: sputtering, laser ablation, e-beam and thermalevaporation.
 3. The method according to claim 1, wherein applying avoltage bias comprises applying a voltage ranging from about 0 volts toabout −500 volts.
 4. The method according to claim 1, wherein depositinga layer of compressive alpha-tantalum comprises sputtering a layer ofcompressive alpha-tantalum having thickness from about 10 Angstroms toabout 4 micrometers.
 5. The method according to claim 1, wherein thesubstrate comprises silicon carbide.
 6. The method according to claim 1,wherein the substrate comprises silicon nitride.
 7. The method accordingto claim 1, wherein depositing a buffer layer comprises depositing alayer of titanium.
 8. The method according to claim 7, whereindepositing a layer of titanium further comprises depositing a layer oftitanium having thickness from about 3 monolayers to about 2000Angstroms.
 9. The method according to claim 7, wherein depositing alayer of titanium further comprises depositing a layer of titaniumhaving thickness from at least about 400 Angstroms.
 10. The methodaccording to claim 7, wherein the layer of titanium orients on thesubstrate with titanium crystal [100] direction perpendicular to thesubstrate.
 11. The method according to claim 7, wherein the layer oftitanium comprises textured titanium grains.
 12. The method according toclaim 7, wherein the lattice matching occurs between the layer oftitanium and the layer of compressive alpha-tantalum.
 13. The methodaccording to claim 1, wherein depositing a buffer layer comprisesdepositing a layer of niobium.
 14. The method according to claim 13,wherein depositing a layer of niobium further comprises depositing alayer of niobium having thickness from about 3 monolayers to about 2000Angstroms.
 15. The method according to claim 7, wherein depositing alayer of niobium further comprises depositing a layer of niobium havingthickness from at least about 200 Angstroms.
 16. The method according toclaim 1, wherein depositing a buffer layer comprises depositing a layerof substantially pure aluminum.
 17. The method according to claim 16,wherein depositing a layer of substantially pure aluminum furthercomprises depositing a layer of substantially pure aluminum havingthickness from about 3 monolayers to about 2000 Angstroms.
 18. Themethod according to claim 1, wherein depositing a buffer layer comprisesdepositing a layer of aluminum-copper alloy.
 19. The method according toclaim 18, wherein depositing a layer of aluminum-copper alloy comprisesdepositing a layer of aluminum-copper alloy comprising up to about 10%by weight of copper.
 20. The method according to claim 19, whereindepositing a layer of aluminum-copper alloy further comprises depositinga layer of aluminum-copper alloy having thickness from about 3monolayers to about 2000 Angstroms.
 21. The method according to claim 1,wherein depositing the buffer layer on the substrate and depositing thelayer of compressive alpha-tantalum on the buffer layer furthercomprises maintaining substrate temperatures less than 300° C.